Mac only: Apple's Mail app can do a lot of things well (after all, it is one of your favorite email clients), but it annoyingly opens everything in a new window. MailTabs is a free plugin that adds tabs to Apple's Mail.app.

Email as a technology has been around for decades, and thanks to wide spread adoption and…
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The plugin is quite simple: just download it, install it, and the next time you open up Mail, you'll see a tab bar below the toolbar. Double-clicking on messages will open them in a new tab instead of a new window. It isn't the prettiest plugin around (the tab's color looks a little awkward since it doesn't match the toolbar), but overall it really makes Mail nicer to use—no longer will you end up with separate windows cluttering up your desktop when you're trying to root through your email. Hit the link to check it out.